Type IA or insulin dependent diabetes (T1D) is caused by an autoimmune process that destroys insulin-producing cells in the pancreatic islets. Although T lymphocytes are known to mediate T1D, success with B cell directed therapy in T1D and other T cell-mediated disorders has led to the recognition that B cells are more important in these diseases than previously thought. Research in this laboratory is focused on function of B lymphocytes that recognize the key cell autoantigen, insulin. Using NOD mice, transgenes from an insulin autoantibody were discovered to fully support the development of T1D while non-insulin binding Ig- transgenes do not. Encounters between circulating insulin and developing anti-insulin B lymphocytes initiates a state of immune tolerance in which the autoreactive B cells remain in the repertoire and present critical epitopes to pathogenic T cells. A specific antibody that specifically targets insulin-binding B cells blocks the progression of T1D in NOD mice. Tracking anti-insulin B cells in a polyclonal repertoire reveals flaws in central tolerance in the bone marrow that is responsible for seeding pathogenic B cells into the repertoire. These findings reveal that B lymphocytes make previously unappreciated contributions the pathogenesis of T1D and suggest a hypothesis that these properties can be exploited to discover new targets for intervention in T1D. This hypothesis will be test in three specific aims. First, anti-insulin B cells that present critical cell epitopes will be used as a source of diabetogenic MHCII molecules on which mass spectrometry will be used to identify actual cell epitopes presented by B lymphocytes. Second, autoantigen specific B lymphocytes will be tested as a different type of target for T1D prevention and as a co-therapy to assist in reversal of diabetes in NOD. Third, the mechanisms of central tolerance that fail in NOD will be identified and repaired using genetic and antibody mediated approaches. Combined these studies will make new discoveries in the NOD mouse model that can be rapidly translated for use in human T1D.